Hitherto, the expression of ThPOK in T cell development was believed to be restricted to the ab T cell lineage, in particular to transitional CD4+8lo thymocytes and SP CD4 cells in the thymus and periphery. We now show that a subset of gdTCR+ thymocytes also expresses ThPOK, and that ThPOK expression correlates largely with expression of the activation marker CD44. Furthermore, ThPOK is expressed in KN6 gdTCR transgenic mice in the presence of high but not low affinity ligands. These observations suggest that ThPOK is induced in gd thymocytes in response to strong TCR signals. Significantly, HD-/- mice, which lack functional ThPOK, show a severe defect in development of mature gd thymocytes, while mice expressing ThPOK constitutively show a striking increase, establishing an important role for ThPOK in maturation and/or lineage commitment of gd cells. Given that the importance of TCR signaling and selection for gd development remains highly controversial, establishing a critical role for ThPOK in these processes would represent a significant advance. Hence in Aims 1 and 2 of the current proposal, we propose to address whether ThPOK induction in gd thymocytes is directly regulated by TCR signaling, and whether ThPOK controls positive/negative selection of gd thymocytes and/or commitment to the gd lineage. Interestingly, while ThPOK appears to be required for normal gd development, its expression in preTCR-expressing DN thymocytes leads to a high incidence of thymic lymphomas. In Aim 3 we propose to identify the cell type and stage in ontogeny at which lymphomagenesis is initiated, and test whether gdTCR expression or gd commitment are sufficient to protect DN thymocytes against ThPOK-mediated lymphomagenesis.